[Aussie Mike]

Panhard rod will be the easiest to fit. On a Cobra where there isn't a huge ammount of suspension travel it will work well. My only thought is that if your rear end uses angled upper control arms to limit side to side movement then the panhard rod may cause some binding with it's slight side to side movement.

Adding weight is never a good option in my opinion. The rear axle assembly is un sprung weight so won't make any difference to the weight balance of your car. What it will do is make life harder for the shocks to control the rear end. I reckon the BTR commodore diff would be a good choice. The whole thing will be lighter than the Ford 9" and plenty strong enough. There are lots of ratios available as well as LSD options.

Cheers